sam3x8e: Add initial support for Arduino Due boards

This adds basic support for running on the Atmel SAM3x8e
micro-controllers that are found in the Arudino Due boards.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/Kconfig

@@ -0,0 +1,25 @@
+# Kconfig settings for SAM3x8e processors
+
+if MACH_SAM3X8E
+
+config BOARD_DIRECTORY
+    string
+    default "sam3x8e"
+
+config MCU
+    string
+    default "sam3x8e"
+
+config CLOCK_FREQ
+    int
+    default 42000000 # 84000000/2
+
+config SERIAL
+    bool
+    default y
+config SERIAL_BAUD
+    depends on SERIAL
+    int "Baud rate for serial port"
+    default 250000
+
+endif